The present invention relates to a method and an apparatus for grinding an elongated knife blade used for a veneer lathe or a veneer slicer. The elongated knife blade is of a length of the order of from one meter to more than three meters.
A veneer lathe is a machine for producing a veneer from a bolt of wood or log. An elongated knife blade is stationarily provided in the veneer lathe and acts on a bolt of wood or log which is gripped between a pair of rotary chucks and rotated around an axis. The knife blade cuts into the peripheral surface of the log as the log is rotated, and peels off a veneer continuously from the log. Such a knife blade can also be used in a veneer slicer. The knife blade must be ground by a grinding wheel to sharpen it after use.
For grinding a knife blade of the above type, it has been a conventional way to apply the grinding wheel to the front or back surface of the cutting edge of the blade and to reciprocate the grinding wheel along and in contact with the cutting edge while the grinding wheel is rotated around its axis. This grinding operation is carried out while the knife blade is mounted in the veneer lathe or veneer slicer, or alternatively after the knife blade is dismounted from the veneer lathe or slicer and is then fixedly mounted on a knife blade mount provided separately from the veneer lathe or slicer.
Before carrying out the grinding operation, either a knife blade mounting stand in front of a knife blade mount of the veneer lathe, or the separate knife blade mount mentioned above is made to have a flat, smooth and level mounting surface on which the knife blade to be ground is fixedly mounted in close surface-to-surface contact throughout its entire length and width.
On the other hand, a grinding liquid (such as water or grinding oil) is ejected to the grinding region which moves along the cutting edge of the knife blade, especially in the case of using the separately provided knife blade mount, because considerable heat is generated during the grinding operation due to high speed rotation of the grinding wheel relative to the knife blade and because burning, cracking and other degradation of the blade as a result of such heat generation must be prevented.
By taking the above measures of causing the knife blade mount to have a flat, smooth and level mounting surface and of ejecting the grinding liquid to the grinding region and further by taking a measure of reciprocating the grinding wheel in parallel with the straight cutting edge, the knife blade that has been ground should have a straight shape in the longitudinal direction of the blade. However, in reality, it has not been possible heretofore to obtain a truly straight shape of the knife blade after the grinding.
For this reason it has been a practice to affix the knife blade that has been ground to a knife mount of the veneer lathe, by making the following troublesome adjusting provision. That is, the knife mount is provided with knife blade pushing bolts and knife blade pulling bolts which are oriented transversely to the longitudinal direction of the blade and disposed at intervals in the longitudinal direction of the blade and which act on the blade edge opposite the cutting edge to exert transverse forces to the blade at various positions along the length of the blade. By adjusting the pushing and pulling bolts in different transverse directions and to different degrees, the cutting edge of the blade is made strictly straight even in the case where the ground knife blade is not straight.
The reason why it has not been possible to obtain a truly straight shape of the knife blade even after a grinding in parallel to the cutting edge is as follows. When the knife blade is ground in parallel to the cutting edge while the knife blade is affixed to a flat, smooth and level mounting surface of the mount, the cutting edge will be made straight immediately after the grinding operation but the cutting edge will undergo a thermal deformation in both the longitudinal and transverse directions due to temperature drop after the grinding.
The knife blade is in close surface-to-surface contact with the mounting surface of the knife blade mount during the grinding. Furthermore the knife blade is mounted on the mount in such a manner that longitudinal thermal expansion of the blade is restricted especially at the two ends thereof. Therefore, as the knife blade cools after the grinding, it undergoes such a deformation that a longitudinally intermediate portion thereof becomes concave on the front side, that is, the blade is curved into an arcuate plate shape. Furthermore, as the knife blade cools after the grinding, the blade also undergoes such a deformation that the proximal edge opposite the cutting edge of the blade is more contracted longitudinally than the cutting edge so that the blade is in the shape of a sector, because of a difference in the amount of generated heat between the regions of the proximal and cutting edges and because of the gradually decreasing thickness of the blade toward the cutting edge.
A difference in thermal deformation also occurs in the knife blade mount. The mounting surface of the mount is influenced by the differentially generated temperature of the knife blade thereon. A mounting surface portion in contact with a portion of the blade being ground in which more heat is generated, is more affected than another mounting surface portion in contact with another portion of the blade in which less heat is generated, so that different thermal stresses are produced in the same mounting surface. A further difference in thermal deformation occurs in the knife blade mount in the direction toward and away from the knife blade. That is to say, the mounting surface in contact with the knife blade is subjected to a more thermal expansion than a portion of the knife blade mount away from the mounting surface. Such differences in thermal expansion and stress in the mount are fed back to the knife blade fixed to the mount and cause the knife blade to be deformed into a curved shape in which a central portion with respect to the longitudinal and transverse directions are raised than the other portion.
When the knife blade fixed to such a deformed mount is ground by the reciprocating grinding wheel, the central portion of the blade will be ground more than the other portion so that when the knife blade cools after the grinding, the thickness of the cutting edge will become larger from the central portion toward the longitudinal end portions. This must be avoided.
Apart from the cooling of the knife blade that occurs by supplying a cooling medium to the grinding region, the feed of the grinding wheel to the cutting edge of the knife blade fixed to the mount has heretofore been carried out by rapidly lowering the rotating grinding wheel to a level close to the cutting edge surface, further gradually lowering the grinding wheel while being reciprocated longitudinally of the cutting edge, confirming with the operator's eye either a sound of contact of the grinding wheel with the cutting edge surface or a spark produced at the time of contact of the grinding wheel with the cutting edge surface, and then feeding the grinding wheel into the cutting edge by a preset amount to carry out the grinding while reciprocating the grinding wheel.
As an alternative measure for starting a grinding operation, a method has been proposed in which the grinding wheel being rotated is incrementarily fed toward the knife blade and a detection is made of a variation of the rotational speed of the grinding wheel at the instant the grinding wheel touches the cutting edge surfaces to thereby detect the position of the start of the grinding. This method is disclosed in Japanese Patent Laid Open Publication No. 3-55,151 published in 1991.
According to the above method of lowering the grinding wheel to the cutting edge while being rotated and reciprocated, the grinding wheel takes a zig-zag path while approaching the cutting edge. This necessitates a considerably long time. Moreover, the point at which the grinding wheel first contacts the cutting edge surface is not always a highest portion of the cutting edge surface. If the first contact point is not a highest portion, the grinding wheel will encounter the highest portion later during its movement along the cutting edge surface and will be subjected to an overloading so that the grinding wheel and/or the cutting edge will suffer a damage.
In contrast, the method disclosed in Japanese Patent Laid Open Publication No. 3-55,151 referred to above makes it unnecessary to cause the grinding wheel to take a zig-zag path in approaching the cutting edge surface and can shorten the time but with the above stated disadvantage of not being able to determine the highest portion of the cutting edge surface. Furthermore, the first contact of the grinding wheel with the cutting edge surface for the purpose of confirming the contact of the grinding wheel with the cutting edge surface, causes an excessive grinding of the surface together with a burning and/or degradation of the surface, which necessitates a further grinding.